Defining Drought:Defining Drought:

Donald A. Donald A. WilhiteWilhite, , DirectorDirector

School of Natural School of Natural ResourcesResources

The Challenge for The Challenge for Early Warning Early Warning

SystemsSystems

The Cycle of Disaster ManagementThe Cycle of Disaster Management

Drought vs. Aridity• Aridity is a permanent

feature of climate.• Drought is a temporary

feature of climate, an aberration.

Drought vs. Water Scarcity

• Water scarcity = an excess of water demand over available supply.

• It can result from . . .– Prevailing institutional arrangements, prices– Overdevelopment or over allocation of the water resource

• Indicators = mining of ground water, increasing conflicts between water use sectors, streams becoming intermittent or permanently dry, land degradation

– Scarcity may be a social construct, i.e., product of affluence, expectations

– Altered supply (e.g., climate change)

: a deficiency of precipitation

(intensity) from expected or “normal” that extends over a season or longer period of time (duration) . . .

Meteorological drought

and is insufficient to meet the demands of human activities and the environment (impacts).

Agricultural drought

Hydrological drought

Socio-economic drought

Comparing drought to other natural hazards–

• slow onset, “creeping phenomenon”, a non-event

• difficult to determine drought onset and end

• absence of a precise, universal definition• impacts are nonstructural and spread over

large areas• severity and impacts best defined by

multiple indices and indicators

Comparing drought to other natural hazards–• no consistent methodology for assessing impacts

or a data base for archiving impacts• impacts are complex, affect many people, and

vary on spatial and temporal timescales, multiple and migrating epicenters

• mitigation interventions are less obvious• water shortages increase conflict—regulatory,

legal authority (interstate and transboundaryissues)

• makes monitoring, early warning, impact assessment, response, mitigation, and planning difficult!

Drought is a normal part of climate in virtually all climate regimes but differs in terms of

• Causal factors– Multiple– Regional in scale– Forcing functions not well understood

• Impacts• Institutional structure for monitoring, mitigation,

response, and planning/policy• Societal coping capacity (vulnerability/resilience)• Government policies (e.g., data sharing across

ministries, response measures)• Government/donor response capability

Is drought a departure from normal climate

or a part of normal climate?

Drought is a normal part of climate!

Spatial Extent

Droughts differ in terms of:

Evolution of Drought Types

Natural and Social Dimensions of Drought

RainfallDeficienciesHeat Stress

Meteorological

SoilsCropsRange

LivestockForests

AgriculturalWater SupplySnow DepthRecreation

TourismHydropower

Hydrological

Socio-economic

Decreasing emphasis on the natural event (precipitation deficiencies)

Increasing complexity of impacts and conflicts

Time/Duration of the event

Societal Impact

Increasing emphasis on water/natural resource management

(social factors)

Components of a Drought Early Warning System

• Monitoring networks– Who is the responsible authority?– Multiple networks?

• Data retrieval systems• Data quality control• Data analysis—converting data into information• Interaction with end users/stakeholders

– Creating an end-to-end-to end system• Information delivery—timely, reliable

Building an effective drought early warning system is like putting together the pieces of a puzzle.

Each index or indicator represents a valuable piece of information to assess the severity of drought and its potential

impact on people and the environment. We do not see the full picture until all pieces are in place.

Key Indicators for Monitoring Drought

• climate data (precipitation, temperature)• soil moisture• stream flow• ground water• reservoir and lake levels• snow pack• short, medium, and long range forecasts• vegetation health/stress and fire danger• sectoral impacts

Drought impacts• Droughts have different physical

characteristics• Society is dynamic so each drought event

is superimposed onto society—impacts reflect changing vulnerabilities

• Does your country have a monitoring system for recording drought impacts?

• How do you incorporate impacts into a drought early warning system?

Drought Indices

• Percent of Normal• Deciles• Palmer Drought Severity Index (PDSI)• Surface Water Supply Index (SWSI)• Standardized Precipitation Index (SPI)• Keetch-Byram Drought Index (KBDI)• Vegetation Condition Index (VCI)• And the list goes on . . . .

Summary Points• Drought is a creeping phenomenon with no universal

definition—definitions are region and application(impact) specific.

• Many indices and indicators are available to assist in the quantitative assessment of drought severity.

• Drought impacts are a key indicator of vulnerability.• Data sources are varied and require interagency

cooperation to assess drought severity, impacts, and mitigation strategies (institutional barrier).

• Stakeholder involvement is a critical component of drought early warning & preparedness planning.

• Early warning systems are the foundation of effective drought mitigation/preparedness plans.

Percentage change in average duration of longest dry period, 30-year average for 2071-2100 compared to that for 1961-1990.

Drought projections for IPCC‘s A1B scenarioWhere we’re headed: Droughts

©2009 The Board of Regents of the University of Nebraska. All rights reserved.

Thanks!

dwilhite2@unl.eduSchool of Natural Resources

snr.unl.edu